1. Field of the Invention
The present invention relates to methods of forming resistance thermometers that are easily made in a batch process and provide the desired characteristics.
2. Description of the Prior Art
Thin film platinum resistance thermometers are gaining acceptance, but there is a problem with obtaining the desired temperature coefficient of resistance (TCR) of the thermometer, and also, in order to obtain the desired nominal resistance, for example 100 ohms, a platinum resistance element formed as a strip or ribbon of substantial continuous lengths made in a serpentine pattern of side by side strip sections. Making a thermometer which has a desired overall small size is difficult. The physical dimensions of the thermometer must be precisely defined so that individual strip or ribbon sections can be placed close to each other, and the platinum strip or ribbon should have a uniform grain from edge to edge, and be free of contaminants to maximize the resistivity of the thermometer.
A process for producing a resistance thermometer element is disclosed in United Kingdom patent application Ser. No. 2,096,645, published Oct. 20, 1982. Magnetron sputtering under selected atmospheres is disclosed. The thermometer strip deposited is laser trimmed or scribed to define the size of the resistive strip. Controlling the atmosphere is emphasized in this publication in order to obtain a satisfactory thermometer.
Diehl et al. U.S. Pat. No. 4,072,593, issued on Feb. 7, 1978, discloses a process for reduction in size of a resistance element for resistance thermometers. A platinum layer is sputtered onto an insulating solid, using a commercial RF sputtering apparatus and including forming "meanders" by means of a laser beam so that the resistance was standardized to 100 ohms. This patent discloses methods of sputtering platinum onto a substrate, but does not suggest the use of a path defined in a separate liftoff material to obtain a desired resistance thermometer. The thermometer, after being formed with the desired pattern by a laser beam (or by photo etching) is tempered or heat treated.
Liu U.S. Pat. No. 4,181,755, issued on Jan. 1, 1980 discloses a way of generating a thin film pattern using a self lifting technique. The process involves depositing a continuous layer of circuit film onto the, surface of the substrate, and then a photo resist pattern coincident with the ultimately desired circuit film pattern is defined on the surface of the circuit film. The portion of the circuit film not covered by photo resist is then removed. While retaining the photo resist and the remaining circuit film, the entire surface of the substrate is coated with a barrier film. The photo resist is then stripped away from the surface of the circuit film causing the barrier film that covers the circuit, to liftoff, causing the circuit film to be exposed. The circuit film pattern is then surrounded by a barrier film pattern. The device is used for making a Schottky barrier diode. In this instance, the barrier film is lifted off from a previously deposited conductor, and the patent does teach or suggest defining a platinum resistive path in an inert liftoff medium, depositing platinum material in the path and then etching away the liftoff medium to leave the thermometer in place.
Symersky U.S. Pat. No. 4,353,935, issued on Oct. 12, 1982 discloses a method of manufacturing a device having a conductor pattern that utilizes a liftoff technique to define at least one conductor. A multi layer mask is required, and in the preferred embodiment the masking layer uses three sublayers of different materials, but it does provide recesses in the masking layer in the form of the conductive pattern that is to be provided. Etching is used, to expose part of the body surface through the masking layers, and then the conductive layer is deposited on both the masking layer and the exposed surface. The patent does not disclose the use of an inert material as a liftoff medium in which a thermometer pattern is formed and which liftoff material is removed after the platinum thermometer is deposited.
Bertram et al. U.S. Pat. No. 4,085,398, issued on Apr. 18, 1978 discloses a thin film resistance temperature detector using a copper mask, which is not useful in the present process, which requires an inert liftoff material. Other prior art of interest includes Diehl et al. U.S. Pat. No. 4,103,275, issued on Jan. 25, 1978 which discloses ionic etching for obtaining a resistance element for resistance thermometers, and Frank U.S. Pat. No. 4,129,848, issued on Dec. 12, 1978 which discloses a platinum film resistor device which has a layer of quartz deposited upon an insulative substrate, which is sputter etched to produce etch pits in the surface for defining a path of the platinum thermometer. Chemical etching to expose the platinum for sputter etching away excessive platinum is disclosed.
Reichelt U.S. Pat. No. 4,050,052, issued on Sept. 20, 1977, also discloses a temperature measuring resistor structure, comprising a resistance thermometer, having a strip of platinum applied to a carrier in a predetermined pattern, and discloses depositing the platinum strip while the temperature of the substrate is in the range of 500.degree.-900.degree. C. This patent also discloses heat treatment, but does not disclose any way of obtaining a sharply defined path as with the present process.
The prior art methods all tend to introduce impurities and cause the loss of definition at the edges of the platinum, thus affecting the TCR of the thermometer. For example, laser fabricating tends to melt the edges of the strip; photo etching gives impurities; ionic and sputter etching silicon or silicon dioxide causes deposition of this material into the platinum as impurities, and the platinum structure is affected or damaged at its edges so that there is a loss of definition. Use of masking in present techniques to etch away the platinum film in unwanted areas is a problem because the pattern cannot reasonably be defined with desired precision and uniformity for obtaining a small size and close spacing of the strip sections. The present masking layers tend to deteriorate before the process is complete and impurities tend to be introduced into the platinum.